Study on the association and phase separation behavior of surfactants and promethazine hydrochloride: impact of ammonium electrolytes

In the current study, the association and phase separation of cationic tetradecyltrimethylammonium bromide (TTAB) and nonionic Triton X-100 (TX-100) surfactants with promethazine hydrochloride (PMH) were investigated in aqueous ammonium-based solutions. The micellization nature of the TTAB and PMH drug mixture was examined by evaluating critical micelle concentration (CMC) and counterion binding extent (β) at different salt contents and temperatures (298.15–323.15 K). Micelle formation in the TTAB + PMH mixture was enhanced in the presence of ammonium salts, whereas the process was delayed with an increase in temperature in the respective salt solution. With an increase in salt content, the cloud point (CP) of the TX-100 + PMH mixture decreased, which revealed that the respective progression occurred through the salting out phenomenon. In micellization and clouding processes, the changes in free energies ΔG0m and ΔG0c were found to be negative and positive, respectively, demonstrating that the corresponding processes are spontaneous and non-spontaneous. Standard enthalpies (ΔH0m/ΔH0c) and standard entropies (ΔS0m/ΔS0c) for the association and clouding processes, respectively, were also calculated and discussed. The core forces amid TTAB/TX-100 and PMH in the manifestation of electrolytes are dipole–dipole and hydrophobic forces among the employed components according to the values for ΔH0m/ΔH0c and ΔS0m/ΔS0c, respectively.


Introduction
][3][4][5][6][7][8][9][10] These molecules are composed of two parts: (i) a hydrophobic component that prefers oil to water and (ii) a hydrophilic component that prefers water to oil.2][13][14][15][16][17][18] Natural and spontaneous processes, such as self-assembly and selforganization, mostly depend on non-covalent interactions (van der Waals, H-bonding, hydrophobic/hydrophilic, and electrostatic). 19The potential of micelle solutions as useful molecular assemblies for applications in various pure and practical sciences has nowadays gained much attention from researchers.Micelle solutions can serve as models for several biochemical and pharmacological systems.As a result, the hydrophobic cores of surfactant aggregates preserve their ability to solubilize water-insoluble compounds. 20n this study, we plan to evaluate the solution properties, association nature, and physico-chemical variables of the mixture of promethazine hydrochloride (PMH) (Scheme 1) and tetradecyltrimethylammonium bromide (TTAB)/nonionic Triton X-100 (TX-100) (Scheme 2) in different ammonium salt media (NH 4 Cl, NH 4 NO 3 , (NH 4 ) 2 SO 4 , (NH 4 ) 2 HPO 4 , and (NH 4 ) 2 -SO 4 $FeSO 4 $6H 2 O, (AFS; ammonium ferrous sulfate)).AFS has been used as a food preservative in analytical chemistry as a substitute for ferrous sulphate.The modication of metabolites such as NH 3 , glucose (C 6 H 12 O 6 ), lactate (C 3 H 5 O 3 − ), and glycine (C 2 H 5 NO 2 ) in the human body is directly inuenced by liver diseases. 21The ammonium ion (NH 4 + ) is generated when ammonia and hydrogen ions combine and are expelled through urine, helping to maintain the normal pH of our bodies.Because the production of NH 4 + plays a noteworthy role in regulating the pH of the human body, the contents of NH 3 change during various diseases, such as liver diseases. 22PMH is a phenothiazine drug used to treat allergies, nausea, and vomiting-related illnesses.Additionally, it is used as a sedative before and aer surgery and to stop a runny nose from a common cold.The present studies of TTAB/TX-100 + PMH in ammonium salt media might provide important knowledge that will be benecial for the progress of drug delivery systems and drug formulations.
2][33] Among them, TX-100 is a fascinating non-ionic surfactant in which an aromatic ring is present in the middle of the structure acting as a bridge between lyophobic polyethylene chains and a lyophilic portion. 22The solubility behavior and pattern of TX-100 are quite amazing.The TX-100 surfactant compound has several notable uses in the production of biopharmaceuticals, including the inltration of cell membranes and the deactivation of lipid-encased viruses.][36][37] Although many works have been reported on the cloudy generation and micellization of surfactants in the manifestation of drugs, [38][39][40][41][42][43][44][45] the behavior of the respective processes in the presence of ammonium-based salts is still unknown.Rub et al. 42 reported the phase partition and assembly of TX-100/TTAB + metformin hydrochloride drug solutions in different hydrotropic media.Ahsan et al. 23 reported the nature of micelle generation and cloudy formation of a sodium dodecyl sulfate (SDS)/TX-100 and lomeoxacin hydrochloride drug mixture.They determined and explored the thermodynamic parameters and interaction forces among them.Recently, we studied the interaction of TTAB/TX-100 with PMH in potassium salts/alcoholic additives. 44,45Herein, we explore the impacts of (i) concentrations of ammonium-based electrolytes on CMC and CP of a working system, (ii) temperature on TTAB + PMH micellization in ammonium salt system; (iii) thermodynamic variables (free energy (DG 0 m /DG 0 c ), enthalpy (DH 0 m /DH 0 c ) and entropy (DS 0 m /DS 0 c )), and (iv) enthalpy-entropy compensation variables of micellization/clouding processes in the attendance of employed salts.

Materials
The procured materials in the working processes were of reagent grades, and these have been applied without any puri-cation.The origin and purity of the materials used are shown in Table 1.

Approach of conductivity
The conductivity measurements were carried out by applying a conductivity meter (Jenway 4510, UK) tted with a dip cell (cell constant = 0.97 cm −1 ) at various compositions and temperatures (298.15  [46][47][48] The desired temperature of the solutions was sustained by utilizing a thermostated water bath (JSRC-13C, Korea).The micelle development concentration (CMC) is determined from the break point acquired in the k vs. [51][52]

Assessment of cloud point (CP)
The solutions containing TX-100 and PMH were formulated in the desired solvent (aq.salt solutions with specic concentrations).In a carefully regulated heating and cooling experiment, the cloud points of solutions were measured visually using the described Albertsson method, 53 which was also adapted by Blankschtein et al. 54 Solutions were placed in a thin Pyrex glass tube, which was then sealed and heated gradually in a water bath with internal circulation and a digital temperature display that had an accuracy of 0.1 K. Near the CP, the temperature increased gradually at a rate of 0.5 K min −1 while being continuously stirred.Visual observation was made of the clouding site at the beginning of the phenomenon.The system was then given time to cool when the heater was turned off.Once more, the temperature was recorded at turbidity clearance.The CP of the system was determined to be the mean of the two temperatures.To determine the right CP values, the procedure was carried out at least 3 times, with the mean of the concurrent run acting as the ultimate CP.The literature 55,56 provides a detailed description of the experimental process.

Results and discussion
3.1.Conductivity measurements 3.1.1.CMC and b of a drug-surfactant system in aq.ammonium salts media.The aggregation of TTAB in the presence of additives is explored in the current investigation from the changes in the conductivity of ionic surfactants in the solution phase.1][52] The hydrophilic and hydrophobic characteristics of the surfactant moiety, the presence of an aqueous additive solution, and environmental factors, such as temperature, affect the CMC value.When the energy produced by the association of the hydrophobic portion of the monomer is adequate to overcome electrostatic repulsion among ionic head groups, the entropy loss that accompanies the aggregation, micellization occurs. 57Herein, the conductivity technique is employed to assess the CMC of the TTAB + PMH system in the presence of ammonium salt media.Three ammonium salts, such as NH 4 Cl, NH 4 NO 3 and (NH 4 ) 2 SO 4 , were used in the current investigation.Fig. 1 demonstrates the change in k with the augmentation of surfactant concentration for the TTAB + PMH mixed solution in aq.ammonium chloride environment.Only a CMC value was observed for the investigated surfactant content for the TTAB + PMH system in the presence of ammonium salts.
In our previous work, 58 the effect of PMH on the assembly of TTAB in an aqueous medium where the addition of the drug favoured the micelle formation of TTAB (the CMC values decreased as a function of PMH concentrations).In that investigation, the used concentrations of PMH were 1, 3, 5, 7 and 10 mmol kg −1 in an aqueous medium, whereas the effect of temperature and addition of K-based salts on the aggregation of the TTAB + PMH mixture was performed considering a 3 mmol per kg PMH drug.The CMC value of 3.71 mmol kg −1 for the assembly of the TTAB + PMH mixture in a water medium was obtained at 310.55 K. 58 The addition of KCl and K 2 SO 4 further facilitated the aggregation of the TTAB + PMH mixture. 58dditionally, doctors prescribe 6.5 mg to 25 mg PMH drug daily, which may vary with the patient's condition.Again, this drug undergoes further dilution when it is administered in the body.Consequently, considering these facts, in the present investigation, 3 mmol per kg PMH drug was kept xed in the entire examination of the micellization of the TTAB + PMH mixture in salt media.The addition of ammonium electrolytes to the solution might affect CMC.This affects the amount of counterion binding, which in turn affects the attraction of the ionic head groups to one another and changes the CMC.The following equation 59,60 is used to empirically quantify this effect: where a and b are two constants for a specic ionic head group and C t represents the entire counterion concentration.The extent of micellization ionization (a) was measured using the following equation: where S 1 and S 2 denote the extent of the slope value of the linear line below and above CMC in the k vs.
[TTAB] plot, respectively.2 and Fig. 2. The CMC values of the TTAB + PMH system experience a reduction in the presence of ammonium salts, and the reducing trend also undergoes augmentation with the enhancing concentration of salts.The CMC of the TTAB + PMH system at 303.15 K in aq.ammonium salts demonstrates the following order: CMC (NH 4 Cl) > CMC (NH 4 NO 3 ) > CMC ((NH 4 ) 2 SO 4 ).The shrinking of the thickness along with electric double-layer potential happens in the manifestation of salts, which reduces the repulsion amongst the charged head groups, thereby generating a favorable environment for the aggregation of surfactant molecules.The (NH 4 ) 2 SO 4 imparts a superior effect to NH 4 Cl and NH 4 NO 3 to produce a satisfactory situation for the assembly.The sulfate ion (SO 4 2− ) is a robust kosmotropic ion with a greater charge density related to the chloride and nitrate ions because these CMC values are lesser than chloride and nitrate ions. 44,62,63owever, the chloride ion is deliberated as a chaotropic identity with a low charge density and has poor power that disrupts water arrangement; thus, the extent of the association of amphiphilic monomers is reduced.We consider a range of concentrations of salts containing chloride, nitrate and sulfate ions containing salts.If the concentration of chloride/nitrate ions and sulfate ions doubles, the CMC values still show a decreasing trend.When 3.031 M NH 4 Cl and 0.9808 M (NH 4 ) 2 SO 4 solutions (both solutions have the same ionic strength (I) of 3 mmol kg −1 ) are used, their CMC values are 3.05 and 2.57 mmol kg −1 respectively.However, when 6.022 M NH 4 Cl solution is used, the CMC is 2.93 mmol kg −1 , which is comparatively higher than the CMC for the (NH 4 ) 2 SO 4 solution of identical ionic strength.
3.1.2.Outcome of the temperature variation on the aggregate formation by the TTAB + PMH system in aq.ammonium salt media.The changes in conductivities with the growing contents of TTAB in the circumstance of the TTAB and PMH mixed solution in aq.system of NH 4 Cl, NH 4 NO 3 and (NH 4 ) 2 SO 4 at study temperatures are discussed in the current investigation.Herein, to inspect the inuences of temperature on the aggregation of TTAB + PMH under identical experimental   hydrations.As the temperature increases, both groups of hydrations tend to diminish. 646][67][68] Thus, the rst factor dominates the second one with the increase in temperature in almost all three ammonium salt solutions used herein in the current study.
where X CMC represents the mole fractional CMC values.The symbols A, B and C are regression constants obtained by the polynomial tting (order 2) of the ln X CMC vs. T plots (Fig. 5), as shown in Table 4.
In aq.ammonium salts, the DG 0 m values of the TTAB + PMH association are acquired to be negative (Table 5).The assembly process turns out to be both enthalpy and entropy guided at higher temperatures in aq.NH 4 Cl and lower temperatures in the cases of aq.NH 4 NO 3 and (NH 4 ) 2 SO 4 media.However, the association is enthalpy controlled in nature in aq.NH 4 NO 3 and (NH 4 ) 2 SO 4 media at higher temperatures.The DH 0 m and DS 0 m values demonstrate that the micellization TTAB + PMH happens through the major contributing forces of exothermic (ion-dipole and dipole-dipole natures) along with hydrophobic interactions. 71,72The extent of hydrophobic interaction is signicantly larger in the H 2 O + NH 4 Cl medium, while the remarkable contribution of exothermic (ion-dipole along with dipole-dipole types) forces exists among components in aq.NH 4 NO 3 and (NH 4 ) 2 SO 4 media.
3.1.4.Enthalpy-entropy compensation in the course of micellization.The computed DH 0 m and DS 0 m values obtained in the case of the assembly of TTAB + PMH mixtures in aq.ammonium salts were used to inspect the compensation phenomena between the enthalpy and entropy of micellization (Fig. 6) by employing eqn (8): 73,74 The compensation temperature (T c ), intrinsic enthalpy gain ðDH 0;* m Þ and R 2 are presented in Table 6.The DH 0;* m values demonstrate the solute-solute interaction and disclose the efficiency of the association of hydrophobic moiety in the course of micelle development. 75,76The higher negative DH 0;* m values refer to the creation of stable drug-induced TTAB micelle.However, the depression of negative DH 0;* m for TTAB + PMH mixture in other studied media suggests a decrease in stability of the PMH-mediated TTAB micelle.The involvement of water in the solution of variable solutes, including proteins, could be assessed from the numerical values of T c values in the range of 270-350 K. 77,78 The T c values of TTAB + PMH solution in the occurrence of ammonium salts are found to be in the range of 293.9-345.4K. Consequently, the detected T c values for the TTAB + PMH mixture in the employed media demonstrate a good comparison with the biological uids.[81] 3.2.Clouding phenomena 3.2.1.Impacts of ammonium-based salts on the phase partitioning manner of TX-100 and PMH drug mixture.The phase partitioning manner of the TX-100 and PMH drug mixture was examined in the manifestation of ammonium salts.In our earlier investigation, the concentration effect of PMH on the cloudy generation of TX-100 in a water medium was inspected taking PMH concentrations of 0.50, 1, 3, 5, 7 and 9 mol kg −1 , while the CP values of the study system increased with the increase in PMH concentration. 82In the current study, a PMH concentration of 1 mmol kg −1 was randomly chosen for the clouding phenomenon.The concentration of PMH was not taken similarly to that of micellization because micellization and clouding generation are different physico-chemical processes.In the current examination, four ammonium salts, such as NH 4         a greater extent of salts with high ionic strength (I) values.The variation in the CP values of TX-100 and PMH solutions in the incidence of ammonium-based salt media is shown in Fig. 7 and Table 7 structure.Owing to the movement and engagement of H 2 O molecules in the creation of salt anions, the nonionic surfactant TX-100 + PMH system experiences "salting out" phenomena, which reduces the entropies of solution systems (Table 7).Water's dipole has a strong attraction to electrolytes, which salts out the molecules of surfactant from the nearby area and increases inter-micellar interaction, thereby promoting phase separation.The ion-dipole interaction experiences an augmentation with an increase in electrolyte contents; consequently, CP values steadily decline as a function of electrolyte content. 22.2.2.Thermodynamics of phase separation.[86]  where X s is the mole fractional solubility of solutes and R and T are the gas constant and CP in kelvin, respectively.The variation in CP can be dened as a function of the logarithmic value of the solute's mole fraction (X s ) by eqn (12).The values of regression constants (A, B and C) in eqn (12) were achieved from 2 nd -order polynomial tting of ln X s against CP plots (Fig. 8), and the values are listed in Table 8.Finally, the DH 0 c of phase separation was obtained from eqn (13).
The values of DG 0 c , DH 0 c and DS 0 c for the TX-100 + PMH mixture in aq.ammonium-salt solvents are presented in Table 9.The positive DG 0 c values were obtained for the creation of cloudy in the current study system.The +DG 0 c values experienced a decrease as ammonium-based salt concentration increases, demonstrating that the amount of non-spontaneity of phase separation undergoes a decrease with the increase in salt concentration.The DH 0 c and DS 0 c values are negative in almost all the experimented cases, demonstrating that the clouding process is exothermic and enthalpy dominated.The negative entropy values disclosed the highly ordered system in aq.electrolyte media.At the highest salt content, the DH 0 c and DS 0 c values become positive.8][89][90] They also described the interaction forces of H-bonding and dipoledipole interaction.The DH 0 c and DS 0 c values for the working system reveal that the principal interaction forces between components are dipole-dipole (exothermic) and hydrophobic interactions.Additionally, there is a possibility of survival of pipi interactions between the drug and surfactant applied for the clouding investigation.

Enthalpy-entropy compensation of clouding phenomena.
In all the circumstances of the current study, excellent linearity between DH 0 c and DS 0 c was obtained (Fig. 9).The respective compensation parameters were determined using eqn ( 14): 91,92 The DH 0;* c and T c values for the current system in aq.ammonium salt environment are summarized in Table 10.4][95] All the analyzed examples had positiveestimated DH 0;* c values, indicating that the system becomes unstable at the stage of cloudy development.The T c values for the TX-100 + PMH mixture fall between 334.1 and 345.6 K.The investigational outcomes show a decent resemblance with the stated T c value for biological uids and tiny solute solutions. 78

Conclusions
This study demonstrates the plausible interactions of TTAB/TX-100 with PMH in aq.ammonium salts media at variable temperatures (ranging from 298.15 K to 323.15 K) and the compositions of salts.The experiments are conducted through conductivity and cloud point measurement tools.A single CMC is detected for the association of TTAB and PMH solution in the presence of different salts.The micelle formation acquired depends on the nature of the salts, their concentration, and the temperature change.The micellization is attained to be augmented in the presence of salts while delaying micelle creation with the increase in the experimental temperature.The CP values of TX-100 + PMH drug decrease in the occurrence of salt, demonstrating the decrease in the solubility of the study system.The magnitudes of free energy of micellization (DG 0 m ) and clouding (DG 0 c ) in the surfactant + PMH mixtures were perceived to be negative and positive, which demonstrated that the processes are spontaneous and non-spontaneous, respectively.In the aq.ammonium salt systems, in most cases, the micellization/clouding of TTAB + PMH mixtures was  exothermic, as shown by the appearance of negative DH 0 m / DH 0 c with exceptions in aq.ammonium chloride solution at lower temperatures (the micellization is endothermic in behavior).In all tested media, the computed DS 0 m /DS 0 c values for the TTAB + PMH mixtures were also observed to be positive/ negative in the respective processes.The DH 0 m /DH 0 c and DS 0 m / DS 0 c values for the TTAB/TX-100 and PMH mixtures in aq.ammonium salt solutions clearly imply that the interactions between surfactants and PMH molecules are most likely to be hydrophobic, ion-dipole/dipole-dipole, and electrostatic forces.There is also a probability of the existence of pi-pi interactions between TX-100 and PMH drug.The important discoveries acquired from this study could be helpful in the industrial formulation of amphiphiles in aq.ammonium salt systems.The solubility nature of the study system in ammonium salts will also impart to the storage and extraction processes.In the future, this work can be expanded to include a wide range of additional potential solvent mixtures with a greater focus on system characterization by utilizing various advanced methods, such as TEM, SEM, ITC, and molecular dynamics simulations.

Fig. 1
Fig. 1 Graph of k vs. [TTAB] for the association of the TTAB + PMH system in aq.NH 4 Cl (3.031 mmol kg −1 ) environment at 303.15 K.

3 . 1 . 3 .
Thermodynamics of micellization.The study of thermodynamics species as a support to identify the nature of the relationship among the used components.The thermodynamic parameters of the TTAB + PMH mixed systems in the presence of ammonium salts are evaluated by employing eqn (3)-(7): 69,70 DG 0 m = (1 + b)RT ln X CMC , The negative values of DG 0 m for TTAB + PMH association in ammonium salts within the working temperature range demonstrate the following order: −DG 0 m (aq.(NH 4 ) 2 SO 4 ) > −DG 0 m (aq.NH 4 NO 3 ) > −DG 0 m (aq.NH 4 Cl).Table 5 also illustrates that at xed salt content the −DG 0 m values are augmented with the growing study temperature.In aq.NH 4 Cl solutions, the DH 0 m values of the TTAB + PMH system are positive and exhibit a decreasing fashion with
concentrations of NH 4 Cl (5.217-256.1 mmol kg −1 ), (NH 4 ) 2 SO 4 (3.334-244.3mmol kg −1 ), (NH 4 ) 2 HPO 4 (5.875-205.8mmol kg −1 ), and AFS (1.502-148.0mmol kg −1 ) were chosen for the investigation, and the respective ionic strengths of the employed ammonium salts are provided in Table7.In the cases of the micellization study, the ionic strength of salts was employed up to 12 mmol kg −1 because of the limitation of the performance of the conductivity meter in salt solutions at high ionic strength.However, in the cloudy creation investigation, the CP values were identied by visual inspection.Additionally, the CP values demonstrate an insignicant change with a slight change in ionic strength; consequently, the study involved

Fig. 7
Fig. 7 Variation in CP as a function of ionic strength (I s ) of employed ammonium-based salts for the TX-100 + PMH drug mixture in aq.ammonium salt solutions.

Fig. 8
Fig. 8 The ln X s vs. CP graph for the clouding of the TX-100 + PMH (1 mmol kg −1 ) mixture an in aq.NH 4 Cl solution.

Table 1
Source and purity of the employed materials used in the experiments

Table 2
CMC and b values of the TTAB + PMH (3 mmol kg −1 ) system in different ammonium salt solutions at 303.15 K

Table 3
the micelle formation of TTAB is delayed with the increase in experimental temperatures.Consequently, the aggregation of the working system occurs with the participation of a larger extent of TTAB as the study temperature increases in the employed salt solution.It is imperative to remark that the CMC values demonstrate almost a plateau in the CMC vs. T plot up to 303.15 K. Later, the CMC values have a habit of growth with an increase in temperatures, and the outcome is acquired in the case of aq.NH 4 NO 3 solution.Two factors signicantly affect changes in CMC values through changes in temperature.The types of hydrations surrounding monomeric amphiphile surfactant molecules and PMH-facilitated TTAB micelles alter as the temperature increases.The aggregated TTAB system exclusively experiences hydrophilic hydration, while the monomeric amphiphile system experiences both hydrophobic (H 2 O arrangement surrounding the non-polar moieties) and hydrophilic (H 2 O organization close to the polar portions) . The CMC values have the propensity to be enhanced with the intensication of temperature in the presence of NH 4 Cl, NH 4 NO 3 and (NH 4 ) 2 SO 4 .The upsurge in CMC for TTAB + PMH micelles in aq.ammonium salt solutions suggests that

Table 3
Different physical parameters for the assembly of amphiphile + drug mixture in aq.ammonium salt media at investigational temperatures

Table 4
Regression constants (A, B, and C) values for TTAB + PMH (3 mmol kg −1 ) aggregation in the aq.solution of ammonium-based salts

Table 5
Values of DG 0 m , DH 0 m , and DS 0 m for the studied system in aq.ammonium salts at different temperatures

Table 6
Values of DH 0;* m and T c of the micellization of the working mixture in aq.salt media © 2024 The Author(s).Published by the Royal Society of Chemistry RSC Adv., 2024, 14, 5981-5993 | 5987

Table 7
CP values of TX-100 + PMH (1 mmol kg −1 ) mixed systems at several ionic strengths (I s ) of the employed ammonium salts 83t was observed that the TX-100 + PMH mixture of CP values decreased as the salt concentrations increased.Above 100 mmol kg −1 concentration of employed salts, the CP values of TX-100 + PMH solutions display the following trend: CP (aq.NH 4 Cl) > CP (aq.(NH 4 ) 2 SO 4 ) > CP (aq.(NH 4 ) 2 HPO 4 ) > CP (aq.AFS).The "salting out" effect is accountable for depressing the CP values of the study system in ammonium salt solutions.The (NH 4 ) 2 -SO 4 $FeSO 4 , (NH 4 ) 2 HPO 4 and (NH 4 ) 2 SO 4 release polyvalent anions in H 2 O solution, and this is why they depressed the CP values to a greater extent.83TheSO 4

Table 8
Values of regression constants (A, B and C) for the TX-100 + PMH (1 mmol kg −1 ) mixture in aq.ammonium salt solvents

Table 10
Values of DH 0;* m and T c of the clouding process of the TX-100 + PMH (1 mmol kg −1 ) mixture in aq.ammonium salt media Medium C salt , mmol kg −1 DH 0;* c ; kJ mol À1 T c (K) R 2